This invention relates to the field of modifying surfaces, specifically modifying surfaces by surface alloying of thin-film coatings using pulsed ion beams.
Many applications require materials having different bulk and surface properties. For example, many industrial applications could benefit from the high corrosion resistance of high purity aluminum. Such aluminum, however, has insufficient tensile strength for practical industrial use. Addition of alloying elements such as silicon, copper, and magnesium can improve the mechanical properties of aluminum, but also can act as initiation sites for corrosion. Addition of transition metals such as titanium, zirconium, hafnium, chromium, and niobium, to aluminum alloys can improve corrosion resistance. These metals, however, are largely insoluble in aluminum under equilibrium conditions.
As another example, titanium and associated alloys have excellent corrosion resistance, but wear resistance is a problem. A thin (about 200 .ANG.) hard layer of TiO.sub.2 forms on titanium under normal atmospheric conditions, but if this layer is penetrated in a wear application, galling and micro-welding can result, leading to a compromise of surface integrity.
Plating of a dissimilar metal onto a surface can be used to change surface properties. Internal stresses of plated layers restrict the attainable thickness, however, and mechanical delamination is a major concern. Delamination or even a small break in the plating can totally compromise performance, for example by allowing environmental contaminants to promote corrosion of the base material under the plated layer.
Electron beams and laser beams can be used to heat a surface and modify its properties. The energy of an electron beam must generally be held to below about 50 kV to produce a suitable melt range. The dynamics of electron beam diodes make it difficult to produce and transport multi-hundred A/cm.sup.2 current densities at 50 kV, limiting the use of electron beams for surface heating and modification. Laser beams are generally inefficient to produce, and couple poorly to highly reflective metal surfaces. Also, laser photons do not penetrate in depth beyond the surface, so thermal diffusion is the dominant energy transfer process. Thermal diffusion leads to a sharply peaked thermal profile, limiting the depth of effective surface modification. Further, laser heating can lead to a rough surface that can compromise manufacturing processes that rely on mechanical tolerances.
Ion implantation has also been used to modify surface properties. Conventional ion implantation uses a continuous low current beam that can not transiently heat surface regions. The depth of surface modification available with ion implantation is typically on the order of 250-1000 Angstroms (.ANG.), precluding the use of ion implantation where deeper surface modification is required.
Pulsed intense ion beams can be used to modify surface properties. See, e.g., Pogrebnyak, "Metastable States and Structural Phase Changes in Metals and Alloys Exposed to High Power Ion Beams," Phys. Stat. Sol. (a) 117, 17, 1990. Pulsed ion beams have also been used in surface alloying of thin-film coatings. See Baglin et al., "Pulsed Proton Beam Annealing: Semiconductors and Silicides," Nucl. Instr. and Meth., 1981. Such research, however, has not developed a consistent regimen for improving mechanical properties such as corrosion resistance and durability. Some research has shown beneficial changes in surface hardness, resistance to corrosion, and surface and near-surface morphology and microstructure. Current pulsed ion techniques that do not include surface alloying do not provide sufficient flexibility with respect to the final composition of the surface since they only modify the microstructure of the existing surface.
Accordingly, there is a need for a surface modification method that can provide beneficial changes in surface properties, can modify a surface to a greater depth than previous methods, and that is suitable for industrial application.